This invention relates to apparatus and methods for detecting regions of lightning activity and more particularly to an apparatus and method for determining and displaying azimuth and range information relating to the occurrence of lightning activity associated with thunderstorms.
Thunderstorms, characterized by turbulence and electrical activity (lightning), create great dangers, particularly to aircraft. It is therefore desirable to locate thunderstorms as accurately as possible so that they can be tracked and avoided. Lightning associated with mature stages of thunderstorms generates electrical signals that propagate through the atmosphere. The detection, recognition, measurement and analysis of these electrical signals provide a basis for storm tracking and avoidance.
Lightning flashes are composed of a series of high current lightning strokes, each stroke being preceded by a lower current discharge called a leader. The duration of electrical activity associated with a lightning stroke varies, but in many instances can last as long as 100 microseconds. The initial rise time of electrical signals associated with a lightning stroke almost never exceeds five microseconds. Following the first peak of the electrical signals of a lightning stroke, lesser signals of sub-microsecond duration, but with fast rise times (of five microseconds or less), will occur.
U.S. Pat. No. 4,422,037, which is the grandparent of the present application, discloses an improved apparatus and method of detecting, tracking and displaying lightning activity. In accordance with the method and apparatus disclosed therein, sensors are provided which measure the time rate of change of the magnetic and electric flux densities of the electrical signals generated by the lightning. The outputs of the sensors are integrated to provide a measure of electric (E) and magnetic (H) fields of the lightning signals. The total magnetic field value is calculated from sampled magnetic fields and the value is compared with a predetermined field strength to estimate in which of several range regions the lightning stroke occurred. The integrated magnetic field value is also compared to predetermined fields strengths to predict in which range region the lightning stroke occurred.
If the regions predicted by the two methods are adjacent regions or the same region, then the ratio of the integrated magnetic to electric field values are used to determine the range from a look up table. If the two regions predicted are the near and distant regions, then the ratio of the total magnetic to total electric field value is calculated from the sampled data and used to determine the range from the look up tables. Due to ambiguities present in the plot of the ratio of magnetic to electric fields, the maximum effective range of such airborne systems in accordance with the invention disclosed in the aforementioned patent is approximately fifty kilometers, at frequencies practical for use in airborne equipment. Lower frequencies could extend this maximum range somewhat, but would necessitate the use of larger sensors which are impractical for installation on smaller aircraft. Considering the high speeds of present day aircraft, this fifty kilometer limitation on range can be quite disadvantageous.
Accordingly, it is an object of the present invention to provide a method and apparatus for detecting and displaying regions of thunderstorm activity at ranges which are suitable for airborne applications.
It is another object of the present invention to provide a method and apparatus for detecting regions of thunderstorm activity which eliminate ranging ambiguity at ranges suitable for airborne applications.
It is yet another object of the present invention to provide a method and apparatus for extending the maximum effective range for detecting thunderstorm activity without increasing the size of sensing equipment.